Various chemical processes generate waste gases containing gaseous pollutants such as organic solvents which are harmful to the human system. From the standpoint of the prevention of air pollution, therefore, a waste gas containing such noxious gaseous pollutants must be freed of the noxious pollutants before it is released into the atmosphere.
Various methods for effecting purification of gases containing harmful gaseous pollutants by adsorption have been devised. As occasion requires, these prior methods involve recovery of the removed noxious pollutants. Of these prior methods, particularly popular is the method which makes use of the so-called fluidized-bed type adsorption system wherein a gas to be treated and adsorbent particles such as activated carbon, activated alumina or silica are brought into mutual contact to form a fluidized bed of the adsorbent particles. In the adsorption treatment of the gas by this fluidized-bed method, it is common practice to effect the gas treatment continuously by having fluidized beds formed in a multiplicity of stages within a tower as illustrated in FIG. 1 of the accompanying drawing, for example. In FIG. 1, 1 denotes a reaction tower. A gas containing noxious gaseous pollutants to be removed is introduced into the tower 1 through a nozzle 2 in the adsorption section A. On entering the tower interior, the gas ascends vertically and comes into contact with adsorbent particles held inside the adsorption section A, causing the adsorbent particles to form fluidized beds on the stepped trays 3, 3', 3" . . . The adsorbent particles forming the fluidized beds adsorb the gaseous pollutants from the gas. The gas which has thus been freed of the noxious pollutants is released into the atmosphere via a discharge outlet 4 at the top of the tower. The adsorbent particles on the stepped trays 3, 3', 3" . . . , fall through the downcommers 5, 5', 5" . . . associated with the trays and descend gradually downwardly by virtue of the gravity, while simultaneously adsorbing the gaseous pollutants from the gas. Then, they leave the adsorption section A and accumulate in the space formed on a funnel-shaped guide plate 6. While they form a gravitationally moving bed in the space, they gradually reach a regeneration section B which is located at the bottom of the reaction tower 1. On entering the regeneration section B, the adsorbent particles are heated by a heater 7, with the result that the particles are regenerated as they are forced by the heating to release the adsorbed pollutants. Subsequently, the regenerated adsorbent particles reaching the bottom 8 of the tower are transferred via a lifting pipe 9 to the top of the tower for recyclic service. In the meantime, the pollutents which have been desorbed from the adsorbent particles are forced out of the system via a nozzle 10 by means of a carrier gas being introduced via a nozzle 11 disposed at the lower portion of the regeneration section B. The discharged pollutents are transferred to a desorbate treating section C composed of a decanter and the like.
In the adsorption treatment of the gas by the fluidized-bed method described above, successful stabilization of the fluidized beds thus formed constitutes an essential requirement for enabling the removal of the noxious gaseous pollutants from the gas to be effected continuously at a high removal efficiency over long periods of service. The stability of such fluidized beds depends on the shape of adsorbent particles used, the strength, wear resistance and other physical properties of the particles and the flow volume, flow velocity and viscosity of the gas used for fluidizing the adsorent particles, and so on. It also depends on the extent of change in the weight of the adsorbent particles being recycled. When the adsorption treatment of gas by the conventional fluidized-bed type technique is reviewed from this point of view, it is noted that the so-called coconut-shell activated carbon obtained from coconut husks is popularly used as the adsorbent particles. The activated carbon of this type is made up of particles of varying, complicated shapes and therefore makes their transport substantially difficult. Moreover, the adsorbent particles have poor physical properties and, for this reason, are readily pulverized as by crushing and attrition. Recyclic use of such activated carbon particles of irregular shapes, therefore, involves numerous difficulties. In the adsorption treatment of gas by the fluidized-bed method, the adsorbent particles of such shapes induce undesirable phenomena such as boiling, channeling and slugging when fluidized by the upward flow of the gas under treatment. They also cause similar phenomena while they are moving downwardly via the downcommers (corresponding to the items denoted by 5, 5', 5" . . . in FIG. 1) by gravity, with the result that smooth flow of the particles inside the is impeded. This impeded flow consequently brings about a quantitative change in the weight of the adsorbent particles being transferred for recyclic service. With a view to precluding these disadvantageous phenomena, the conventional techniques have attempted to improve the structure of downcommers for the particles. It has been suggested, for example, to incorporate orifices in the bottoms of the downcommers or, as disclosed by U.S. Pat. No. 2,674,338, to have bottom plates supported on springs on the bottoms of the downcommers. These attempts at improvement of the structure of downcommers, however, effectively complicate the system itself and have the a disadvantage that activated carbon particles have their shapes vary gradually with the lapse of time. Thus, all these attempts fail to attain the preferred Stabilization of the quantitative transport of adsorbent particles. Because the adsorbent particles in use are highly susceptible to pulverization and also because stabilization of the transportation of these adsorbent particles is difficult to accomplish, the conventional techniques do not easily achieve stabilization of the fluidized beds of the adsorbent particles.